A major problem with any interpolation algorithm which produces a sharp result, i.e. cubic or higher polynomial, or a digital filter design, is that objectionable `ringing`, overshoot and undershoot will occur on high contrast edges in the image. Some degree of overshoot and undershoot is required on lower contrast edges to produce a visually pleasing result but this will produce objectionable `ringing` on a high contrast edge.
The `ringing` can be reduced by various methods. For example, a digital filter design using linear programming can be used to obtain a compromise between a sharp cut-off filter, to obtain a sharp image, and a certain amount of `ringing`. However, since it is a compromise, reducing ringing on a high contrast edge will lead to subjectively less pleasing results in lower contrast areas of an image, i.e. the image will be less sharp overall.
Methods by which different interpolation algorithms can be used during an interpolation process have been suggested. For example, U.S. Pat. No. 4,468,693 describes interpolation hardware including a manually operated switch for switching between different available interpolation algorithms depending on the kind of image to be interpolated. U.S. Pat. No. 5,008,752 describes a system which incorporates a control unit which monitors the content of the input data and switches between interpolation kernels in an attempt to provide optimum interpolation for different types of image content. With this method, certain corrective measures need to be taken in order to compensate for the use of either the one or the other algorithm.
It is an object of the present invention to provide a continuum of interpolated output images ranging from relatively sharp at one end of the continuum to relatively soft at the other end so that the user has the ability to select any degree of sharpness (or softness) in interpolated output images within limits imposed by a predefined set of interpolation algorithm coefficients.
It is a further object of the invention to provide local image content adaptability to allow a user to specify or select the degree of sharpness (or softness) in the interpolated output image based upon local image characteristics, such as density and contrast.
In this specification the terms sharp and soft are used as direct opposites; i.e. 100% sharpness equals softness and vice versa. Furthermore the terms "soft" and "smooth" are used synonymously.